1. Field of the Invention
The present invention relates generally to a piezo film switch, and more particularly to a piezo film switch capable of providing an indication of movement in two opposite directions.
2. Statement of the Prior Art
Piezo film is a flexible, lightweight, tough plastic film which is typically formed of a polarized homopolymer of vinylidene fluoride (e.g., polyvinylidene fluoride or "PVDF"). Piezo film is adaptable to diverse applications because of its piezoelectric nature. KYNAR.RTM. piezo film, available from the assignee of the instant application, is one example of a piezo film.
It is well known that when a working voltage is applied to the electrodes of piezo film, it will elongate or contract, depending on the polarity of the applied voltage. If the film is exposed to an alternating voltage, it will elongate and contract as the polarity changes. It is also well known that when an external force is applied to the film (e.g., compressive or tensile strain), the film develops a proportionate open circuit voltage. Exposure to a reciprocating force results in a corresponding alternating electrical voltage signal.
One application for piezo films is in switches. See, for example, Ben H. Carlisle, "Piezoelectric Plastics Promise New Sensors", Machine Design (Oct. 23, 1986), pp. 105-110. For most applications, a piezo film switch can be made by laminating a piezo film to one surface of a thin, flat spring. When a stress is applied to the film, such as by deflecting the spring, a voltage pulse is provided. The amplitude of the voltage pulse in the piezo film switch is directly proportional to the magnitude of applied stress, and hence to the resulting strain. Moreover, deforming the film by stretching its length will maximize the stress, and therefore the resulting output voltage, since the applied stress acts upon the film's cross-section.
Piezo film switches have distinct advantages over other types of switches. They are not susceptible to malfunction by contaminants such as dirt, moisture and abrasive dust which tend to cause failure in conventional mechanical switches. Since a piezo film switch operates essentially by developing a charge within the film and transferring that charge to the film's outer electrodes, no mechanical closure or opening is required to make or break an electrical contact. Unlike membrane keyboard switches that radiate high frequencies during an electronic scan, piezo film switches operate at low current and generate minimal RF levels. Furthermore, piezo film switches do not experience "contact bounce" and therefore they simplify circuit design.
Additional information relating to the structure, properties, applications and fabrication of piezo film switches, as well as piezo films in general, can be found in the Kynar.RTM. Piezo Film Technical Manual (1987), which is available from Pennwalt Corporation, Philadelphia, Pennsylvania, the assignee of the present application. This manual is incorporated herein by reference.
Most prior art piezo film switches provide better performance when subjected to tensile, rather than compressive, strain. Therefore, these switches have not been utilized in designs requiring sensing of deflection in two directions (compression and tension) with satisfactory results. Applications where such a dual direction switch may be employed include turnstiles, gates, utility meters, pinball machines, etc., i.e., wherever sensing of mechanical motion in two opposite directions is required. The present invention addresses the need for a piezo film switch that can be employed in these applications.